U.S. Pat. No. 5,240,885 (Aitken et al) describes the preparation of rare earth metal-doped cadmium halide glasses, which glasses transmit radiation well into the infrared portion of the electromagnetic radiation spectrum due to their low phonon energy. That capability commended their utility for the fabrication of efficient lasers, amplifiers, and upconverters when doped with the appropriate rare earth metals. Because metal-sulfur bonds are generally weaker than metal-oxygen bonds, sulfide glasses exhibit much lower phonon energies than oxide glasses and, therefore, transmit radiation much further into the infrared region of the electromagnetic radiation spectrum. Accordingly, sulfide glasses were seen to have the potential of being excellent host materials of rare earth metals for applications such as those listed above requiring efficient radiative emission.
Unfortunately, however, many sulfide glasses are black and, consequently, are unsuitable for some of the above applications inasmuch as such a host glass would tend to absorb the pump radiation instead of the rare earth metal dopant. One of the best known sulfide glasses, viz., arsenic sulfide, is transparent to radiation in the long wavelength range of the visible portion of the radiation spectrum as well as far into the infrared region and, hence, would appear to be a suitable host glass for rare earth metals. Nevertheless, rare earth metals have been found to be relatively insoluble in arsenic sulfide glasses, and it has proven to be difficult to dope those glasses with enough rare earth metal for sufficient pump power absorption.
Rare earth metals are known to be very soluble in most oxide glasses and their apparent insolubility in arsenic sulfide glasses has been conjectured to be due to the gross structural dissimilarity existing between the latter and oxide glasses. Arsenic sulfide glasses are believed to consist of long chains and layers of covalently bonded pyramidal AsS.sub.3 groups, whereas oxide glasses typically comprise a three-dimensional network of relatively ionically bonded MO.sub.4 tetrahedra, where M is a so-called network-forming metal such as silicon, phosphorus, aluminum, boron, etc. Rare earth metals are readily accommodated in these ionic network structures where they can compensate charge imbalances that arise from the presence of two or more network-forming metals, e.g., aluminum and silicon in aluminosilicate glasses--energetically similar sites may not exist in the two-dimensional covalent structures typical of arsenic sulfide and related glasses.
One system of sulfide glasses which exhibit good transparency in both the visible and infrared portion of the radiation spectrum, and which possess a relatively ionic three-dimensional structure that would be expected to be more accommodating of rare earth metals, comprises gallium sulfide glasses. In contrast to arsenic sulfide glasses, the structure of these glasses is based upon a three-dimensional linkage of corner sharing GaS.sub.4 tetrahedra. Rare earth metals are readily soluble in these glasses. In fact, some of the most stable gallium sulfide glasses contain a rare earth metal as a major constituent.